1. Field of Invention
The present invention relates to a light guide plate, surface light source device and display. The present invention is applied, for instance, to a display which is incorporated in a portable phone, personal computer, navigation system, in particular, LCD of frontlighting type or backlighting type.
2. Related Art
It is known well to employ a surface light source device to illuminate a LCD panel of a LCD. In general, a surface light source device comprises a light guide plate and a primary light source for supplying light thereto.
While rod-like fluorescent lamps (cold cathode tubes) are currently and broadly in use as primary light sources, those provided with point-like light emitter(s) such as Light Emitting Diode (called LED hereafter) are employed growingly in recent years. In particular, the latter type becomes rather predominant in the uses for LCDs with generally small-size screen such as LCDs for portable phones.
In a such surface light source device, light is introduced into a light guide plate and followed by an inner propagation on the way of which light is emitted from an emission face gradually. That is, the propagation involves chances of inner incidence to the emission face, with the result that some of light overcoming the critical angle condition escapes from the light guide plate through the emission face.
A surface light source device is incorporated, for example, in a backlighting arrangement or frontlighting arrangement of a LCD to illuminates a LCD panel. An outlined cross section view of a prior art LCD with a frontlighting arrangement is illustrated in FIG. 16. The LCD 1 shown in FIG. 1 is composed of a reflection-type LCD panel 2 and a surface light source device 3 disposed in front of the LCD panel.
While the surface light source device 3 keeps lighting-on, the LCD panel 2 is supplied with light from the surface light source device 3. Beside this, ambient light transmits through the surface light source device 3 to be supplied to the LCD panel 3. Therefore, if the LCD is used in a well-lighted circumstance, the surface light source device 3 is able to provide an image display without lighting-on.
The light incident to the LCD panel 2 contributes the display operation according to a well-known principle. Since the LCD panel 2 belongs to a reflection-type, a reflection light to provide an image display travels from the LCD panel 2, after transmitting through the surface light source device 3 to the ambiance.
The surface light source device 3 comprises a light guide plate 4 made of a transparent plate-like member having a uniform thickness and a primary light source 5 disposed close to a side face of the light guide plate. The primary light source 5 may be a rod-like fluorescent lamp or point-like light emitter(s) such as LED(s). Illumination light L supplied by the primary light source 5 is introduced into the light guide plate 4 through an end face 4A (called xe2x80x9cincidence facexe2x80x9d, hereafter) to become an inner propagation light.
Illumination light L, after becoming an inner propagation light undergoes reflections repeatedly at a major face 4B on the LCD panel 2 side (called xe2x80x9cemission facexe2x80x9d, hereafter) and at a back face 4C opposite with the emission face 4B while propagating as to be distant gradually from the incidence face 4A. Such light propagating within the light guide plate 4 as to be distant gradually from the incidence face 4A, in other words, as to approach a distal end face 4F is called xe2x80x9cforward inner propagation lightxe2x80x9d in the instant specification.
As shown in a partially enlarged illustration A, slopes 4D are formed repeatedly on the back face 4C as to extend approximately in parallel with the incidence face 4A. The slopes 4D have inclinations determined so that a forward propagation light L is converted into an inner reflection light which is directed to a direction roughly frontal with respect to the emission face 4B.
Between slopes 4D, 4D neighbouring each other, inner propagation relay faces 4E connecting the slopes each other are formed in order to avoid thickness of the light guide plate 4 from falling toward the distal end face 4F. As illustrated, the inner propagation relay faces 4E are flat faces gently inclined inversely as compared with the slopes 4D, reflecting an inner propagation light as to give a further advance. Inclination angle of the inner propagation relay faces 4E is, for example, about 4 degrees.
It is noted that xe2x80x9cinclination anglexe2x80x9d is expressed here as inclination angle with respect to the emission face. Such a small inclination of the inner propagation relay faces 4E leads to almost no light leaking from them.
On the other hand, a forward inner propagation light L inner-impinges to the slopes 4D directly or after being inner-reflected by any inner propagation relay face 4E. In order to produce an inner reflection light directed to the emission face 4B effectively, it is desired that inner reflections at the slopes 4D satisfy the total reflection condition as possible. Taking this standpoint into consideration, each slopes 4D has an inclination angle about 45 degrees. An inner-reflections at a slope 4D brings a roughly frontal emission from the emission face, which is supplied to the LCD panel 2.
However, the above-described prior art tends to bring an uneven brightness on some portion of the emission face 4B. In particular, this problem is strongly apt to arise in cases where the primary light source 5 utilizes a point-like light emitter such as LED. FIG. 17 illustrates an uneven brightness appearing in such a case.
Referring to FIG. 17, a light guide plate 4 employed in a surface light source device, for example, for a LCD incorporated in a portable phone has an incidence face 4A at a center portion of which LED 6 is arranged. The LED emits light which becomes a forward inner propagation light as fore-mentioned and is inner-reflected by slopes formed on a back face (not shown) as to be emitted from an emission face 4B.
Since such an emission promotion is effective for only forward inner propagation light, a shortage of emission occurs in regions where an insufficient forward inner propagation light reaches, in particular, in corner portions ARC indicated by hatching. In other words, the corner portions ARC are dark.
A method of resolving the above-mentioned problem has been proposed. According to the method, a light guide made of a transparent rod-like member is arranged along an incidence face to convert emission from a LED into a line-like illumination light. And this illumination light is introduced into the light guide plate. This method requires a surface light source device to have a complicated structure.
An object of the present invention is to provide a light guide plate which is simply-structured and improved so that an even brightness is realized on an emission face thereof. Another object of the present invention is to provide a surface light source device which is improved so that an even illumination output is obtained. A still another object of the present invention is to provide a liquid crystal display which is improved so that an evenly bright display screen.
Further, viewing from a different standpoint, the present invention aims at a light guide plate improved as to be strikingly suitable for cases where a primary light source utilizing a point-like light emitter is employed, at a surface light source device employing the light guide plate and at a liquid crystal display employing the surface light source device.
In the first place, the present invention is applied to a light guide plate comprising an emission face for outputting light, a back face opposite with said emission face, an incidence face for inputting light and a distal end face located opposite to said incidence face.
According to the present invention, at least one of said back face and said emission face provides an emission promotion surface to urge a backward inner propagation light to be emitted preferentially from said emission face. The term xe2x80x9cbackward inner propagation lightxe2x80x9d means light that is produced by a reflection at said distal end and then travels as to approach said incidence face, and this term makes a pair with xe2x80x9cforward inner propagation lightxe2x80x9d which has been defined already.
It is noted that xe2x80x9curge a backward inner propagation light to be emitted preferentiallyxe2x80x9d means xe2x80x9can backward inner propagation light is urged to be emitted more intensively as compared with forward inner propagation lightxe2x80x9d.
In general, backward inner propagation light has undergone a larger distance travelling as compared with a forward inner propagation light. Such a long travelling causes the light to be expanded and spread widely within the light guide plate.
Therefore, an emission promotion effected after a long travelling causes the emission face to provide a uniformalized emission intensity (brightness). After all, a preferential emission promotion of a backward inner propagation light is advantageous for obtaining a uniformalized emission intensity (brightness). It is preferable for reinforcing the backward inner propagation light to dispose a reflector as to be in contact with the distal end of the light guide plate.
According to a typical example of emission promotion surface, said back face is provided with a great number of slopes inclinations of which are determined so that a backward inner propagation light is converted into an inner reflection light which is directed to the emission face, thereby making said back face provide an emission promotion surface. Light input to the light guide plate may be performed by a primary light source consisting of a point-like light emitter.
In the next place, the present invention is applied to surface light source device including a light guide plate and a primary light source, the light guide plate comprising an emission face for outputting light, a back face opposite with said emission face, an incidence face for inputting light and a distal end face located opposite to said incidence face, and said primary source supplying light to said light guide plate through said incidence face.
According to the present invention, at least one of said back face and said emission face provides an emission promotion surface to urge a backward inner propagation light to be emitted preferentially from said emission face. It is noted that the backward inner propagation light is produced by a reflection at said distal end and then travels as to approach said incidence face.
According to a typical example of emission promotion surface, said back face is provided with a great number of slopes inclinations of which are determined so that a backward inner propagation light is converted into an inner reflection light which is directed to the emission face, thereby making said back face provide an emission promotion surface. Light input to the light guide plate may be performed by a primary light source consisting of a point-like light emitter.
Further, the present invention is also applied to liquid crystal display equipped with a surface light source device for illuminating a LCD panel. The surface light source device includes a light guide plate and a primary light source, the light guide plate comprising an emission face for outputting light, a back face opposite with said emission face, an incidence face for inputting light and a distal end face located opposite to said incidence face, and said primary source supplying light to said light guide plate through said incidence face.
According to the present invention, at least one of said back face and said emission face provides an emission promotion surface to urge a backward inner propagation light to be emitted preferentially from said emission face. It is noted that the backward inner propagation light is produced by a reflection at said distal end and then travels as to approach said incidence face.